pxeboot - Workaround BIOS breakage

pxeboot - Workaround BIOS breakage

* Some BIOSes (the one in my Gigabyte H97N-WIFI) continue to use the original
  negotiated by the TFTP preloader even after we have negotiated a new IP
  and used it in the udpopen.  This will cause our dest_ip filter in
  udpread() to implode.  Detect and handle this case.

* Why does dhcpd hand out different IPs?  I tracked one down to the PXE
  BIOS sending a Client-ID option.  However, our PXEBOOT DHCP and our
  dhclient don't seem to send that option and also get different IPs.

* This won't fix post-boot breakage if dhclient rips the IP out from under
  the NFS root mount in a pxebooted situation.